Process control systems require the accurate measurement of process variables. Typically, a primary element senses the value of a process variable and a transmitter develops an output having a value that varies as a function of the process variable. For example, a level transmitter includes a primary element for sensing level and a circuit for developing an electrical signal proportional to or representing sensed level.
Knowledge of level in industrial process tanks or vessels has long been required for safe and cost-effective operation of plants. Many technologies exist for making level measurements. These include magnetostrictive, capacitance, ultrasonic and microwave radar, to name a few.
One form of process instrument is of the intrusive type in which the primary element is in direct contact with the process fluid for sensing level. A magnetostrictive transmitter is an example of an intrusive type level measurement instrument. A magnetostrictive transmitter has a probe including a magnetostrictive wire maintained under tension in a tube. The probe extends into the process vessel. A magnetic float is movable proximate the probe and floats atop the fluid in the vessel. An electrical pulse is transmitted on the magnetostrictive wire. The electrical pulse interacts with the magnetic field of the float, which creates a torque on the wire to produce a torsional force on the wire, thus initiating a torsional wave that propagates along the wire at the speed of sound. This is known as the Wiedemann effect. Typically, a pickup sensor is positioned at one end of the wire to sense the torsional wave on the wire. The elapsed time is measured between the launch of the electrical pulse and the signal from the pickup sensor. The distance between the magnet and the pickup sensor is calculated from the measured elapsed time multiplied by the speed of the torsional wave, representing level.
The signal from the pickup sensor is a relatively low level signal. Thus, the transmitter must generally be mounted as close as possible to the probe. Typically, the probe is an integral component of the magnetostrictive transmitter and they are assembled and tested together during manufacturing.
Advantageously, the transmitter would be removable from the probe and also be remote mountable. In fact, it would be advantageous to replace a transmitter, if necessary, without removing the probe from the process vessel. However, the flexibility to remove the transmitter from the probe runs the risk that calibration of the system will be corrupted when the transmitter is not placed on the probe for which it was originally calibrated. The calibration is typically stored in the transmitter and accounts for variations in the performance of the probe. When the probe and the transmitter are always inseparable, calibration is not an issue. Allowing the transmitter to be removed from the probe runs the risk of poor performance because of the incorrect calibration.
The performance of the system is also dependent on how far the transmitter's amplifiers are separated from the pickup sensor of the probe. The magnetostrictive sensor generates very small signals that can be corrupted by electromagnetic interference. This issue is made worse by spacing the circuitry further from the pickup sensor.
This application is directed to improvements which allow a transmitter to be removable and/or remote mountable.